Wireless communication systems often employ power amplifiers for increasing the power of a signal. In a wireless communication system, a power amplifier is usually designated as the last amplifier in a transmission chain (the output stage) and it is the amplifier stage that typically requires the most attention to power efficiency. Indeed, the performance of a transceiver in a wireless communication system depends primarily on the performance of the power amplifier. High gain, high linearity, stability, and a high level of power-added efficiency (i.e., ratio of the difference between the output power and the input power to DC power) are characteristics of an ideal amplifier.
In general, a power amplifier operates at maximum power efficiency when the power amplifier transmits peak output power. However, power efficiency worsens as output power decreases. Recently, the Doherty power amplifier technique has been the focus of attention not only for base stations but also for mobile terminals because of its high power-added efficiency.
A Doherty power amplifier typically includes two or more amplifiers, for example, a “carrier amplifier” and a “peaking amplifier.” The amplifiers are connected in parallel with their outputs joined by an offset transmission line, which performs impedance transformation. The peaking amplifier delivers current as the carrier amplifier saturates, thereby reducing the impedence seen at the output of the carrier amplifier. Thus, the carrier amplifier delivers more current to the load while it is saturated because of a “load-pulling” effect. Since the carrier amplifier remains close to saturation, a Doherty power amplifier is able to transmit peak output power so that the total efficiency of the system remains high.
The high efficiency of the Doherty architecture makes it desirable for current and next-generation wireless systems. However, it presents unique challenges in terms of semiconductor package design. Current Doherty power amplifier semiconductor package design calls for the utilization of discrete devices and integrated circuits, for example, one that forms the carrier amplifier and another that forms the peaking amplifier. These discrete devices are maintained a distance apart in order to limit problems with crosstalk that can occur between the carrier and peaking amplifiers. One source of crosstalk in the semiconductor package architecture is between arrays of signal wires, referred to herein as bondwire arrays, that may be utilized between electrical devices in each of the carrier and peaking amplifiers. That is, the performance of a Doherty power amplifier can be adversely perturbed by coupling (i.e., the transfer of energy from one circuit component to another through a shared magnetic or electric field) between adjacent bondwire arrays of the corresponding components of the Doherty power amplifier. Coupling can be of two types, electric (commonly referred to as capacitive coupling) and magnetic (used synonymously with inductive coupling). Inductive or magnetic coupling occurs when a varying magnetic field exists between current carrying parallel conductors that are in close proximity to one another, thus inducing a voltage across the receiving conductor.
Unfortunately, maintaining spatial distance between amplifiers, for example, the carrier and peaking amplifiers of a Doherty power amplifier, to control crosstalk caused by inductive coupling limits the miniaturization of the semiconductor package. Limiting the miniaturization of such devices is undesirable where low cost, a low weight, and a small volume are essential for application.